1. Field of the Invention
This invention is in the field of medicinal chemistry. In particular, the invention relates to conformationally constrained mimetics of Smac which function as inhibitors of Inhibitor of Apoptosis Proteins. The invention also relates to the use of these mimetics for sensitizing cells to the induction of apoptotic cell death.
2. Related Art
The aggressive cancer cell phenotype is the result of a variety of genetic and epigenetic alterations leading to deregulation of intracellular signaling pathways (Ponder, Nature 411:336 (2001)). The commonality for all cancer cells, however, is their failure to execute an apoptotic program, and lack of appropriate apoptosis due to defects in the normal apoptosis machinery is a hallmark of cancer (Lowe et al., Carcinogenesis 21:485 (2000)). Most of the current cancer therapies, including chemotherapeutic agents, radiation, and immunotherapy, work by indirectly inducing apoptosis in cancer cells. The inability of cancer cells to execute an apoptotic program due to defects in the normal apoptotic machinery is thus often associated with an increase in resistance to chemotherapy, radiation, or immunotherapy-induced apoptosis. Primary or acquired resistance of human cancer of different origins to current treatment protocols due to apoptosis defects is a major problem in current cancer therapy (Lowe et al., Carcinogenesis 21:485 (2000); Nicholson, Nature 407:810 (2000)). Accordingly, current and future efforts towards designing and developing new molecular target-specific anticancer therapies to improve survival and quality of life of cancer patients must include strategies that specifically target cancer cell resistance to apoptosis. In this regard, targeting crucial negative regulators that play a central role in directly inhibiting apoptosis in cancer cells represents a highly promising therapeutic strategy for new anticancer drug design.
Two classes of central negative regulators of apoptosis have been identified. The first class of regulators is the Bcl-2 family of proteins, as exemplified by two potent anti-apoptotic molecules, Bcl-2 and Bcl-XL proteins (Adams et al., Science 281:1322 (1998); Reed, Adv. Pharmacol. 41:501 (1997); Reed et al., J. Cell. Biochem. 60:23 (1996)). Therapeutic strategies for targeting Bcl-2 and Bcl-XL in cancer to restore cancer cell sensitivity and overcome resistance of cancer cells to apoptosis have been extensively reviewed (Adams et al., Science 281:1322 (1998); Reed, Adv. Pharmacol. 41:501 (1997); Reed et al., J. Cell. Biochem. 60:23 (1996)). Currently, Bcl-2 antisense therapy is in several Phase III clinical trials for the treatment of solid and non-solid tumors. Several laboratories are interested in designing small molecule inhibitors of Bcl-2 and Bcl-XL.
The second class of central negative regulators of apoptosis is the inhibitor of apoptosis proteins (IAPs) (Deveraux et al., Genes Dev. 13:239 (1999); Salvesen et al., Nat. Rev. Mol. Cell. Biol. 3:401 (2002)). IAP proteins potently suppress apoptosis induced by a large variety of apoptotic stimuli, including chemotherapeutic agents, radiation, and immunotherapy in cancer cells.
X-linked IAP (XIAP) is the most potent inhibitor in suppressing apoptosis among all of the IAP members (Holcik et al., Apoptosis 6:253 (2001); LaCasse et al., Oncogene 17:3247 (1998); Takahashi et al., J. Biol. Chem. 273:7787 (1998); Deveraux et al., Nature 388:300 (1997); Sun et al., Nature 401:818 (1999); Deveraux et al., EMBO J. 18:5242 (1999); Asselin et al., Cancer Res. 61:1862 (2001)). XIAP plays a key role in the negative regulation of apoptosis in both the death receptor-mediated and the mitochondria-mediated pathways. XIAP functions as a potent endogenous apoptosis inhibitor by directly binding and potently inhibiting three members of the caspase family of enzymes, caspase-3, -7, and -9 (Takahashi et al., J. Biol. Chem. 273:7787 (1998); Deveraux et al., Nature 388:300 (1997); Sun et al., Nature 401:818 (1999); Deveraux et al., EMBO J. 18:5242 (1999); Asselin et al., Cancer Res. 61:1862 (2001); Riedl et al., Cell 104:791 (2001); Chai et al., Cell 104:769 (2001); Huang et al., Cell 104:781 (2001)). XIAP contains three baculovirus inhibitor of apoptosis repeat (BIR) domains as well as a C-terminal RING finger. The third BIR domain (BIR3) selectively targets caspase-9, the initiator caspase in the mitochondrial pathway, whereas the linker region between BIR1 and BIR2 inhibits both caspase-3 and caspase-7 (Salvesen et al., Nat. Rev. Mol. Cell. Biol. 3:401 (2002)). While binding to XIAP prevents the activation of all three caspases, it is apparent that the interaction with caspase-9 is the most critical for its inhibition of apoptosis (Ekert et al., J. Cell Biol. 152:483 (2001); Srinivasula et al., Nature 410:112 (2001)). Because XIAP blocks apoptosis at the down-stream effector phase, a point where multiple signaling pathways converge, strategies targeting XIAP may prove to be especially effective to overcome resistance of cancer cells to apoptosis (Fulda et al., Nature Med. 8:808 (2002); Arnt et al., J. Biol. Chem. 277:44236 (2002)).
Although the precise role of XIAP in each type of cancer is far from completely understood, evidence is mounting to indicate that XIAP is widely overexpressed in many types of cancer and may play an important role in the resistance of cancer cells to a variety of current therapeutic agents (Holcik et al., Apoptosis 6:253 (2001); LaCasse et al., Oncogene 17:3247 (1998)).
XIAP protein was found to be expressed in most of the NCI 60 human cancer cell lines (Tamm et al., Clin. Cancer Res. 6:1796 (2000)). Analysis of tumor samples in 78 previously untreated patients showed that those with lower levels of XIAP had significantly longer survival (Tamm et al., Clin. Cancer Res. 6:1796 (2000)). XIAP was found to be expressed in human malignant glioma (Wagenknecht et al., Cell Death Differ. 6:370 (1999); Fulda et al., Nature Med. 8:808 (2002)). XIAP was found to be expressed in human prostate cancer cells and blocks Apo2 ligand/tumor necrosis factor-related apoptosis inducing ligand-mediated apoptosis of prostate cancer cells in the presence of mitochondrial activation (McEleny et al., Prostate 51:133 (2002); Ng et al., Mol. Cancer. Ther. 1:1051 (2002)). XIAP is overexpressed in non-small cell lung cancer (NSCLC) in patients and has been implicated in pathogenesis of NSCLC (Hofmann et al., J. Cancer Res. Clin. Oncol. 128:554 (2002)). Expression of XIAP and lack of down-regulation of XIAP upon treatment with cisplatin have been implicated in cisplatin resistance of human ovarian cancer (Li et al., Endocrinology 142:370 (2001); Cheng et al., Drug Resist. Update 5:131 (2002)). Taken together, these data suggest that XIAP may play an important role in resistance of several human cancers to current therapeutic agents.
Apoptosis is not a single process, rather, it is involved with a number of different, sometimes interconnected, signaling pathways leading to cell degradation. The pathways involved in a particular form of apoptosis depend on many factors, such as the insult or insults that initiate the process. Other factors include the activation or overactivation of specific receptors, such as the activation of “death” receptors by tumor necrosis factor alpha (TNFα), tumor necrosis factor-related apoptosis-inducing ligand (TRAIL or Apo2L), or FAS ligand. Another determining factor is the type of cell which is involved, since different signaling pathways are shown for so called type I and type II cells after Fas or TNFα receptor activation.
TRAIL (Apo2L) has been shown to be a selective and potent inducer of apoptosis in cancer cells (but not normal cells) upon binding to either of two pro-apoptotic TRAIL receptors, TRAIL R1 (or DR4) (Pan et al., Science 276:111 (1997)) or TRAIL R2 (KILLER, or DR5) (Wu et al., Nat. Genet. 17:141-143 (1997); Pan et al., Science 277:815 (1997); Walczak et al., EMBO J. 16:5386 (1997)). Activation of the pro-apoptotic death receptors by TRAIL induces the formation of death inducing signaling complex (DISC), which consists of receptor FADD as an adaptor (Kischkel et al., Immunity 12:611 (2000); Kuang et al., J. Biol. Chem. 275:25065 (2000)), and caspase-8 as an initiator caspase. Once DISC is formed, caspase-8 is auto-processed and activated by induced proximity (Medema et al., EMBO J. 16:2794 (1997); Muzio et al., J. Biol. Chem. 273:2926 (1998)).
TRAIL has generated significant interest as a potential cancer therapeutic (French et al., Nat. Med. 5:146 (1999)) because of its selective targeting of cancer cells, whereas most normal cells appear to be resistant to TRAIL (Ashkenazi et al., Science 281:1305 (1998); Walczak et al., Nat. Med. 5:157 (1999)). Systemic administration of TRAIL has proven to be safe and effective at killing breast or colon xenografted tumors and prolonging survival in mice (Walczak et al., Nat. Med. 5:157 (1999)). Although TRAIL can specifically kill many types of cancer cells, many others display TRAIL-resistance (Kim et al., Clin. Cancer Res. 6:335 (2000); Zhang et al., Cancer Res. 59:2747 (1999)). In addition, cancer cells have been killed by application of antibodies (monoclonal or polyclonal) that specifically recognize either TRAILR1 or TRAILR2.
Numerous mechanisms have been identified as potential factors responsible for TRAIL-resistance. Such mechanisms exist at a number of levels, including at the receptor level, mitochondria level, post-mitochondria level, and at the DISC level. For example, loss of caspase-8 expression (Teitz et al., Nat. Med. 6:529 (2000); Griffith et al., J. Immunol. 161:2833 (1998)), or high expression of the cellular FLICE inhibitor protein (cFLIP) (Kim et al., Clin. Cancer Res. 6:335 (2000); Zhang et al., 1999; Kataoka et al., J. Immunol. 161:3936 (1998)) make cancer cells resistant to TRAIL. Yeh et al. have shown that cFLIP-deficient embryonic mouse fibroblasts are particularly sensitive to receptor-mediated apoptosis (Yeh et al., Immunity 12:533 (2000)). Several splice variants of cFLIP are known, including a short splice variant, cFLIP-S, and a longer splice variant, cFLIP-L. It has been shown that cFLIP-deficient embryonic mouse fibroblasts become resistant to TRAIL-induced apoptosis as a result of retroviral-mediated transduction of cFLIP-S (Bin et al., FEBS Lett. 510:37 (2002)).
Although TRAIL represents a particularly promising candidate for tumor-selective death receptor activation (i.e., it induces apoptosis preferentially in tumor cells but not in normal tissues), many cancer cells are resistant to apoptosis-inducing drugs, as discussed above. As a result, treatment with such drugs often requires co-treatment with irradiation and/or cytotoxic chemicals to achieve a therapeutic effect. However, both radiation and chemotherapy have significant side effects, and are generally avoided if possible.
Thus, a need exists for an agent that can selectively and efficiently sensitize tumor cells for apoptosis-inducing drugs such as TRAIL or TRAIL receptor antibodies, without also sensitizing the surrounding normal cells. Such an agent would be useful for reducing or preventing the drug resistance commonly associated with the use of receptor-mediated apoptotic cancer drugs, thus improving their effectiveness and eliminating the need for combination therapies.
Recently, Smac/DIABLO (second mitochondria-derived activator of caspases) was identified as a protein released from mitochondria into the cytosol in response to apoptotic stimuli (Budihardjo et al., Annu. Rev. Cell Dev. Biol. 15:269 (1999); Du et al., Cell 102:33 (2000)). Smac is synthesized with an N-terminal mitochondrial targeting sequence that is proteolytically removed during maturation to the mature polypeptide. Smac was shown to directly interact with XIAP and other IAPs and to disrupt their binding to caspases and facilitate caspase activation. Smac is a potent endogenous inhibitor of XIAP.
High resolution, experimental three-dimensional (3D) structures of the BIR3 domain of XIAP in complex with Smac protein and peptide have recently been determined (Sun et al., J. Biol. Chem. 275:36152 (2000); Wu et al., Nature 408:1008 (2000)) (FIG. 1). The N-terminal tetrapeptide of Smac (Ala-Val-Pro-Ile, or AVPI (SEQ ID NO:1)) recognizes a surface groove on the BIR3 domain of XIAP through several hydrogen-bonding interactions and van der Waals contacts. The interaction between BIR3 and caspase-9 has also been shown to involve four residues (Ala-Thr-Pro-Phe, or ATPF (SEQ ID NO:2)) on the amino terminus of the small subunit of caspase-9 to the same surface groove on the BIR3 domain. Several recent studies have convincingly demonstrated that Smac promotes the catalytic activity of caspase-9 by competing with caspase-9 for the same binding groove on the surface of the BIR3 domain (Ekert et al., J. Cell Biol. 152:483 (2001); Srinivasula et al., Nature 410:112 (2001)).
Unlike most protein-protein interactions, the Smac-XIAP interaction is mediated by only four amino acid residues on the Smac protein and a well-defined surface groove on the BIR3 domain of XIAP. The Kd value of Smac peptide AVPI (SEQ ID NO:1) to XIAP (Kd=0.4 μM) is essentially the same as the mature Smac protein (Kd=0.42 μM). This well-defined interaction site is ideal for the design of non-peptide, drug-like small molecules that mimic the binding of Smac to XIAP.
A cell permeable Smac peptide, which consists of the first four amino acid residues (AVPI (SEQ ID NO:1)) of the N-terminus of Smac tethered to a carrier peptide to facilitate intracellular delivery, was recently shown to sensitize various tumor cells in vitro and malignant glioma cells in vivo to apoptosis induced by death receptor ligation or cytotoxic drugs (Fulda et al., Nature Med. 8:808 (2002)). Importantly, this Smac peptide strongly enhanced the anti-tumor activity of Apo2L/TRAIL in an intracranial malignant glioma xenograft model in vivo. Complete eradication of established tumors and survival of mice was only achieved upon combined treatment with Smac peptides and Apo2L/TRAIL. Of significance, Smac peptide does not have detectable toxicity to normal brain tissue.
A second recent independent study also showed that peptides consisting of the first four to eight amino acid residues of the N-terminus of Smac tethered to a different carrier peptide enhanced the induction of apoptosis and the long term anti-proliferative effects of diverse chemotherapeutic drugs, including paclitaxel, etoposide, SN-38, and doxorubicin in MCF-7 and other human breast cancer cell lines (Arnt et al., J. Biol. Chem. 277:44236 (2002). This study conclusively showed that XIAP and cIAP-1 are the primary molecular targets for these peptides in cells.
A third study showed that a Smac peptide of the first seven N-terminal residues tethered to polyarginine restored the apoptosome activity and reversed the apoptosis resistance in non-small cell lung cancer H460 cells (Yang et al., Cancer Res. 63:831 (2003)). XIAP was shown to be responsible for the defect in apoptosome activity and suppression of caspase activity in H460 cells. When used in combination with chemotherapy, the cell-permeable Smac peptide regressed the tumor growth in vivo with little toxicity to the mice. Taken together, these recent independent studies strongly suggest that a potent, stable, cell-permeable Smac mimetic may have great therapeutic potential for the treatment of human breast cancer and other types of cancer.
Peptide-based inhibitors are useful tools to elucidate the anti-apoptotic function of IAPs and the role of IAPs in response of cancer cells to chemotherapeutic agents. But peptide-based inhibitors in general have intrinsic limitations as potentially useful therapeutic agents. These limitations include their poor cell-permeability and poor in vivo stability. Indeed, in these three published studies using Smac-based peptide inhibitors, the peptides had to be fused to carrier peptides to make them relatively cell-permeable.
To overcome the intrinsic limitations of peptide-based inhibitors, the present invention involves the design of non-peptidic mimetics based upon Smac peptide and the high resolution experimental three dimensional structures of Smac in complex with XIAP BIR3 domain.